173 Scientific Prospectus

INTRODUCTION

In the Iberian Abyssal Plain Ocean Drilling Program (ODP) Leg 149 defined landward and oceanward limits to the crustal ocean-continent transition (OCT), where the transition zone is defined as the region between a margin-parallel peridotite ridge marking the landward edge of ocean crust and the most seaward tilted fault block of continental crust. However, only one hole (Hole 900A) penetrated basement in the 130-km-wide region between these limits. Leg 173 (Fig. 1) is a sequel to Leg 149, and it will enable (1) drilling and coring of a well-imaged major detachment fault (an analogue to the S-reflector), (2) recovery of more rift-related igneous material (e.g., gabbro) and its host rock (mantle), continental crust or slow spreading oceanic crust, (3) testing of the nature of the topographic high between Site 900 and the most landward known serpentinite basement outcrop, and finally (4) sampling of the oldest oceanic crust. These observations, together with the improved quality and quantity of seismic images, will allow us to address the modes of breakup of the lithosphere, the timing and nature of melt generation from the mantle during the final stages of rifting and, the nature and age of early-formed ``normal'' oceanic crust. The planned drilling will also add to our knowledge of the early sedimentary history of the rifted margin.